COMPETITION, FACILITATION, AND COMPENSATION FOR INSECT HERBIVORY IN AN ANNUAL SALT MARSH FORB

Herbivore impacts on plants are predicted to be greatest in competitive and stressful environments. Plant neighbors can exert strong facilitative as well as competitive effects, however, and facilitation is often mediated through the reduction of environmental stresses. Thus, neighbors may simultaneously increase plant compensation for herbivory, by reducing environmental stress, and reduce compensation by competing for resources. To determine whether aboveground competition and facilitation influence the ability of an annual forb, Atriplex patula var. hastata , to compensate for herbivory, I performed a field experiment in which the presence of insect herbivores and plant neighbors were manipulated in a factorial design. The neighbor treatments included removal of dominant rushes (cleared), rushes left intact (vegetated), and rush removal plus reduction of environmental stress by shading of the soil surface (buffered). In cleared treatments, herbivory reduced survival by almost 50% and reproduction by 37%. In contrast herbivory had no significant effects on plant survival or reproductive output in buffered treatments. Thus, facilitation through soil shading increased plant compensation for herbivory. However, herbivores also strongly reduced fruit production (45%) in vegetated treatments, suggesting that competition magnifies herbivore impacts. The results demonstrate that neighboring plants can simultaneously increase and decrease plant tolerance to herbivory.